Conventionally, it has been practiced to conduct intake air into each cylinder via two separate passages, and to provide a first intake control valve in the first passage and a second intake control value in the second passage. In particular, the timing of opening one of the two intake control valves may be shifted to that of the other so that the performance of the engine may be optimized over the entire rotational speed range. See Japanese patent laid open publication No. 63-201336 (patent document 1). In the previously proposed system, the two intake control valves are opened one after the other as the rotational speed of the engine is increased.
When two intake control valves are used, it is important to be able to accurately determine the intake air flow rate of each intake control valve so that a desired optimum engine control may be performed. In Japanese UM publication No. 5-17402 (patent document 2), it is disclosed to detect an opening angle of the intake control valve, consisting of a butterfly valve, for instance, and estimate the intake air flow rate from the detected opening angle of the intake control valve.
The changes in the intake pressure immediately downstream of an intake control valve (or the intake volume for each cylinder or torque output) for a given change in the opening angle of the intake control valve increases when the engine rotational speed is reduced, and decreases when the engine rotational speed is increased. As the first intake control valve is gradually opened from a small opening angle, the increase rate of the intake pressure drops and the intake pressure converges to a certain value (or reaches a saturated state) once a critical pressure is reached. The converged value is lower as the engine rotational speed is increased. The opening angle of the first intake control valve corresponding to the converged value (effective opening angle) is greater as the engine rotational speed is increased.
Conventionally, the second intake control valve is opened when the opening angle of the first intake control valve has reached a prescribed opening angle (such as a full open angle) in a sequential manner. Therefore, the second intake control valve may not be opened although the opening angle of the first intake control valve has reached a converged value at which the increase rate of the intake pressure is minimal or the intake pressure has saturated. In this region, the vehicle operator feels that the engine torque does not to respond to the depression of the accelerator pedal. In other words, the vehicle operator feels that the vehicle does not accelerate even though the vehicle operator steps on the accelerator pedal, and this may impair the handling or the impression of the vehicle. This occurs not only when accelerating the vehicle but also when decelerating the vehicle.